In the copending applications above referred to, it is pointed out that, in all electrophotographic processes of the prior art in which a developed image was transferred to a carrier sheet, such transfer was effected by bringing the carrier sheet into contact with the developed image on the surface bearing the developed image. I will describe my invention in respect of latent images formed on a photoconductor by photography. It is understood, however, that my invention is applicable to an electrostatic image formed on a surface and then transferred to a carrier sheet such as paper.
In the methods of the prior art, liquid developing compositions are the simplest and would normally produce the greatest resolution, since the toner particles of dry toner developers are larger. Unfortunately, in the liquid systems of the prior art, when the developed image is contacted with a carrier sheet for transfer, the image tends to get squashed or flattened. As a consequence, the thickness of the image--that is, the height of the toner on the image thickness--had to be substantially reduced in order to diminish the squashing effect of contact transfer and the resulting loss of resolution or edge sharpness. When the thickness of the image is reduced, there is a lessened quantity of pigment in the image, which produces a low-density image. Three main disadvantages are present in the liquid-toned transfer ("LTT" hereinafter) method. They are as follows:
(a) In the image area, the squashing demands a very thin image which has a low density. This results in requiring a very smooth paper or other medium as a carrier sheet. Rough surfaces may have an amplitude of roughness which is greater than the thickness of the toned image, but the result is that only the tips of the carrier sheet receive the image. PA1 (b) Since the LTT process normally requires wetting of the entire photoconductive surface with a liquid developer, the non-image areas become moistened with the carrier liquid. As a result, there is evaporation of the carrier liquid, which is usually a low-boiling hydrocarbon. This is disadvantageous, from the standpoint of possible pollution in a closed area, and a waste of costly carrier liquid. PA1 (c) When contact transfer is made, dust, fibers, and other contaminants from the carrier sheet are left on the photoconductive surface. These are wiped or washed into the toner liquid remaining in the machine. PA1 (a) They are relatively non-reactive; PA1 (b) they are manufactured under known conditions, so I do not have the presence of unknown materials in the toning composition; and PA1 (c) surfactants are not used in their manufacture, so I do not encounter any surface-active materials which would affect the toner.
All of these disadvantages can be overcome by the gap transfer process--that is, the transfer of a liquid-developed image across a gap to a carrier sheet. First, I am enabled to have a much thicker and pigment-rich--that is, polymer-rich--developed image. Such an image, if allowed to contact the carrier sheet, would be squashed, with the result that resolution and sharpness would be greatly diminished. I am enabled, by gap transfer, to make a very thick developed image and, since I am transferring the image across an air gap, there is virtually no limitation to the thickness of the developed image because of the dimensions of the air gap itself. Of course, there are other constraints, such as the electrostatic fields, the maximum charge the photoconductive surface would hold depending on its dark resistance, the charge to mass ratio, and other considerations.
The maintaining of an air gap by spacing the photoconductive surface from the means for holding the carrier sheet mechanically is so difficult that it is substantially unfeasible. The tolerance of the air gap must be maintained within tens of microns or less. The dimensions of the air gap depend on the tolerance of the photoconductive drum, its concentricity, the uniformity of thickness of the photoconductive surface, the thickness of the paper, and variations in dimension depending on the coefficients of expansion of the materials involved. It will be seen that the essence of my invention is the maintenance of an air gap by dispersed means located between two planes--that is, between the surface of the photoconductor and the surface of the carrier sheet.
In the copending applications above identified, there are disclosed three methods for maintaining an air gap irrespective of variations in tolerance between the paper and the photoconductor. In copending Application Ser. No. 149,539, I have shown means carried by the paper, such as deformations in the paper surface, or plastic bumps, or other means carried by the paper to form the gap. In copending application Ser. No. 249,336, E. Paul Charlap and I have shown means for dusting the developed image with spacer particles or forming deformations on the photoconductive surface to produce the gap. In copending application Ser. No. 250,720, I have shown a composition in which the spacing means comprise spacer particles carried by the developing composition. In copending application Ser. No. 267,465, I show an improvement in which the spacer particles have a surface charge of the same polarity as the charge of the toner particles and a dielectric constant greater than the dielectric constant of the carrier liquid and in which the toner particles have a low charge to mass ratio so as to enable them to form flocs. Since the spacer particles have a surface charge of the same polarity as the charge of the toner particles, they will codeposit with the toner particles dispersed throughout the developing liquid.
In order to prevent the non-image areas from contacting the developer-wetted photoconductor, it is necessary to interpose spacing means between the photoconductor and the non-image areas. To prevent squashing the image, the spacer particles must codeposit with the toner. This means that the spacer particles must bear the same charge as the toner particles. For example, in the case of a selenium-tellurium photoconductor, the corona charge is positive, so the toner particles must be negatively charged. I had no difficulty in having spacer particles codeposit with toner particles. One difficulty which arose, however, was in having the spacer particles deposit on the non-image areas. To do this, one would expect that the spacer particles for the non-image areas should be positively charged. This, however, cannot succeed because positively charged spacer particles would almost instantly be coated with negatively charged toner particles. This produces black dots on the non-image areas. I found the solution to the problem was to have two separate disciplines function in respect of the spacer particles. One discipline has already been described--that is, electrophoresis. The other discipline is to permit polarization of essentially neutral spacer particles or even spacer particles which are slightly charged either positively or negatively. The polarization forces can be orders of magnitude more powerful than surface charge forces. I have described, in copending application Ser. No. 267,465, that I can cause deposition of the spacer particles in the non-image areas by applying a field across the metering gap which removes excess liquid toner from the developed image. The spacer particles respond to the field intensity, since they are made of polarizable material. They preferably have a higher dielectric constant than the carrier medium. Since the particles are polarizable by the field in the metering area, as described in copending application Ser. No. 267,465, they deposit in the non-image areas by a dielectrophoretic force. It will be seen that charged spacer particles will move to the image areas by electrophoresis, while neutral or slightly charged spacer particles will move to the non-image areas by dielectrophoresis following polarization.
Unfortunately, the spacer particles codeposited with the toned image on the carrier sheet form a powdery image. Spacer particles tend to move and, accordingly, scratch the image when they roll about. Furthermore, the number of charged spacer particles which are removed from the dispersion in the liquid toner composition is a function of the overall image area and the density. If there are large black areas in the image, a large amount of charged spacer particles will be removed from the liquid composition. One solution to the problem of eliminating the powdery feel of the developed image, wherein the spacer particles become detached, is to coat that portion of the spacer particles which is to go to the image with toner. Those particles will then form part of the image and give the image a rich feel, almost as if the image were embossed. One difficulty I encountered with this solution of the problem was that the coated spacer particles would settle in a photocopying machine, for example, when the machine was not in use. The ideal spacer particles, both for those which move dielectrophoretically to the non-image areas and those which move to the image areas electrophoretically, would be those which have the same specific gravity, or slightly less specific gravity, than the specific gravity of the dispersing liquid phase of the developing composition. I have solved this problem by making the spacer particles of hollow beads--preferably out of glass--though any beads, such as hollow phenol-condensation product beads, hollow carbon beads, and hollow aluminum beads, all perform successfully. Glass beads have certain advantages--namely:
Since the uncoated beads--that is, the neutral or dielectrophoretic beads--do not go to the image, the depletion of these spacer particles is negligible. The depletion of the coated microsphere of microballoon spacer particles is such that it must be corrected. This is done by adding coated spacer particles, from time to time, to correct the progressive depletion. Feeling the transferred copy is a good indication of the necessity of adding coated spacer particles. When a sufficient population of spacer particles is present, the copy has an embossed feel; that is, the transferred copy feels raised from the carrier such as paper. That is to say, one senses a distinct thickness of the printed area. When this feel diminishes, it is time to add a quantity of coated spacer particles. If there are insufficent spacer particles present, a contact of the image with the carrier sheet will occur, with the result that the copy will be blurred and the resolution diminished.